Method for producing semi-conducting devices and devices obtained with this method

ABSTRACT

A semi-conducting device has at least one layer doped with a doping agent and a layer of another type deposited on the doped layer in a single reaction chamber. An operation for avoiding the contamination of the other layer by the doping agent separates the steps of depositing each of the layers.

[0001] The present invention relates in general to the domain ofsemiconductor films based on silicon technology. It concerns, moreparticularly, a method for producing silicon junctions, doped or not,which can be used, for example, in solar cells. It also concerns anyother semi-conducting devices obtained by such a method.

[0002] Amorphous or microcrystalline silicon solar cells are made ofmultilayer systems where semiconducting material with certainelectronical and physical properties is deposited, layer by layer, on asubstrate.

[0003] The n-layers and p-layers are doped with other elements toachieve desired properties, such as electrical conductivity. Moreprecisely:

[0004] p-doped layers have a surplus of positive charge carriers,

[0005] n-doped layers have a surplus of negative charge carriers, and

[0006] i layers are intrinsic.

[0007] Generally, boron is used as the doping agent of the p-layers andphosphor as the doping agent of the n-layers.

[0008] Silicon solar cells manufacturers use either single-chamber ormulti-chamber reactors to produce commercial photovoltaic (PV) modules.Plasma deposition of silicon solar cells in a single-chamber reactorleads to considerable simplifications and reduced costs as compared tomulti-chamber processes.

[0009] However, in a single chamber deposition process of a p-i-n solarcell, for example, the subsequent deposition of the i-layer on thep-layer may cause boron recycling from the reactor walls and from thedeposited p-layer. As a result, boron will contaminate the i-layer atthe critical p-i interface and thereby weaken the strength of theelectrical field in the i-layer close to p-i interface. This provokes aless efficient carrier separation just in this zone and leads to areduced collection efficiency in the solar cell and thereby to adeterioration of the cell performance.

[0010] For that reason, most silicon p-i-n solar cells modules are, atpresent, deposited using multi-chamber reactors. Boroncross-contamination by recycling is avoided by simply depositing thep-layer and the i-layer in different chambers. However, the higherinvestment in multi-chamber systems equipment becomes a drawbackparticularly in the field of solar cells where costs are a major issue.

[0011] Similar problems exist with n-i-p solar cells in which phosphorused to dope the n-layer contaminates the i-layer at the critical n-iinterface.

[0012] Thus, an interesting solution would be to combine a lowcost-single chamber reactor with a process scheme able to suppress theboron or phosphor cross-contamination.

[0013] Different treatments have been tested with encouraging results,but they still leave open the question of the light-induced degradationof these solar cells, they use expensive gases, they have long treatmentdurations or are incompatible with large area deposition in industrialreactors.

[0014] The object of the present invention is to provide a method forproducing semiconductors with a particular application in solar cells,avoiding cross-contamination by doping agents and exempt ofdisadvantages above mentioned.

[0015] More precisely, in order to achieve these goals, the inventionconcerns a method for producing a semi-conducting device comprising atleast a layer doped with a doping agent and a layer of another typedeposited on said doped layer in a single reaction chamber. Thedeposition steps of said layers are separated by an operation foravoiding the contamination by the doping agent of said another layer.

[0016] Advantageously, the operation comprises a dosing of the reactionchamber with a compound able to react with the doping agent.

[0017] According to a first embodiment, the contamination avoidingoperation comprises a dosing of the reaction chamber with a vapour orgas comprising water, methanol, isopropanol or another alcohol.

[0018] According to a second embodiment, the contamination avoidingoperation comprises a dosing of the reaction chamber with a vapour orgas comprising ammonia, hydrazine or volatile organic amines.

[0019] The invention also concerns a semi-conducting device comprisingat least a layer doped with a doping agent and a layer of another typedeposited on said doped layer. The interface between said layerscontains traces of oxygen or of nitrogen as a result of a treatment foravoiding the contamination of said another layer by the doping agent.

[0020] Other characteristics of the invention will be shown in thedescription below, made with regard to the attached drawing, where:

[0021]FIG. 1 shows the reactor used for the implementation of themethod, and

[0022]FIG. 2 illustrates the effect of the doping agent contaminationavoiding operation.

[0023] The following description is particularly related, as an example,to the production of a boron doped p-i-n junction, i.e. a semiconductordevice comprising respective p, i and n layers successively deposited ona suitable substrate providing the base of a solar cell.

[0024] The three layers are deposited in a manner well known by a personskilled in the art but, according to the invention, the method comprisesan important supplementary step.

[0025]FIG. 1 shows the reactor used to produce such a semi-conductingdevice. Basically, it comprises:

[0026] a vacuum chamber 10 connected to a vacuum circuit 11,

[0027] a hot wall inner chamber 12 disposed inside the vacuum chamber10,

[0028] a radio-frequency-powered electrode 13 placed inside the innerchamber 12, and

[0029] a showerhead 14 incorporated within the electrode 13 andconnected to different gas feeding lines to introduce appropriatereacting products.

[0030] A substrate 15, for example a glass/TCO substrate of the typeAsahi U, based on SnO₂:F (glass coated with fluorine doped SnO2), isbeing arranged in the inner chamber 12.

[0031] The above described installation is preferably adapted from theindustrial KAI™-S reactor of Unaxis Displays in order to constitute aPlasma Enhanced Chemical Vapour Deposition (PECVD) system. The typicaldimensions of the inner chamber 12 are 50 cm width×60 cm length×2.5 cmheight.

[0032] For the initial p-layer deposition on substrate 15, the reactinggas introduced in the reactor through the showerhead 14 are, typically:

[0033] to form the p-layer: silane, methane and hydrogen, and

[0034] to dope the layer with boron : trimethylboron (TMB).

[0035] TMB is particularly well suited, instead of diborane (commonlyused) because it has a superior thermal stability in the hot reactor andis reported to cause less contamination.

[0036] To perform the deposition of the p-layer, the plasma excitationfrequency used is e.g. 40.68 MHz, the temperature is 200° C., while thepressure is kept at 0.3 mbar, and the power RF is applied at a level of60 W.

[0037] Many experiments have suggested that boron introduced in thereactor is not simply present in a gaseous state which could be easilypumped out, but might be physisorbed on the internal reactor surfacesand desorb very slowly after a pumping period.

[0038] Therefore, according to a first embodiment of the invention,after the deposition of the p-layer and before the deposition of thei-layer, the internal surfaces of the reactor and the substrate also aredosed with a vapour or a gas comprising water, methanol or isopropanolor another alcohol.

[0039] More precisely, in this example, the dosing product is stored ina separate bottle 21 connected, via a valve 22, to the vacuum chamber10, which is kept at low pressure condition. When the valve 22 isopened, the dosing product starts boiling in the bottle 21 because ofthe low pressure inside and vapour flushes into the chamber 10. Of-course, the RF electrode 13 is off. The operation is performed between100 and 350° C., typically at 200° C. and during less than 10 minutes,typically 2 minutes and at 0.05 to 100 mbar. The flow of water vapourhas to be sufficient. For example, 90 mbar.sec is a good value. Ifmethanol or isopropanol is used, the flow is generally higher.

[0040] After the dosing operation, a short pumping period of less than 5minutes, typically around 3 minutes, under similar conditions butwithout any dosing gas addition, is advantageously respected before thedeposition of the i-layer.

[0041] As a result of the above dosing operation, the boron which wasphysisorbed on all the internal surfaces of the reactor and of thesubstrate is transformed into stable chemical compounds unable todesorb. A contamination of the layer which will be later deposited onthe p-layer is thus avoided.

[0042] After this treatment, the i-layer, then the n-layer are depositedin the same reactor. The conditions described above for the p-layerdeposition are reused with appropriate reacting gases, as known by aperson skilled in the art.

[0043] As an example, the reacting gases used for the deposition of thei-layer are a mix of 75% of silane and 25% of hydrogen, whereas thereacting gases used for the deposition of the n-layer are silane,hydrogen and phosphine.

[0044] The evaluation of the base level boron contamination of thei-layers can be made by Secondary Ion Mass Spectroscopy (SIMS) in orderto trace the boron concentration depth profile across the p-i interface.

[0045] To illustrate the efficiency of the above-described dosingtreatment, FIG. 2 shows, as an example, the boron SIMS profile (depth Xfrom surface in Angstroms versus boron concentration Y in atoms.cm⁻³) ofa p-i-p-i sandwich structure deposited on a c-Si wafer. Both p-dopedportions 17 and 18 are normally deposited.

[0046] A first i-layer 19 is deposited on the p-layer 17 withoutperforming any additional treatment. The base level contamination ofboron measured in the i-layer 19 is about 10¹⁸ atoms.cm⁻³.

[0047] A second i-layer 20 is deposited on the p-layer 18 portion afterthe dosing treatment as described above. The base level contamination ofboron measured in the i-layer 20 is reduced to about 10¹⁷ atoms.cm⁻³,which represents an improvement of one order of magnitude.

[0048] The boron contamination in the i-layer of a solar p-i-n celltreated according to the invention can also be indirectly detected byperforming voltage dependent quantum efficiencies measurements as wellas monitoring the global cell performance especially the fill factor ofthe solar cell. The results are substantially the same as those obtainedwith cells deposited in multi-chamber reactors.

[0049] Furthermore, an oxygen peak can be observed with a SIMS analysisat the treated p-i interface, meaning that the above described treatmenthas been used. Typically, the amount of oxygen in the peak is higherthan 10¹⁹ atoms.cm⁻³.

[0050] According to a second embodiment of the invention, after thedeposition of the p-layer and before the deposition of the i-layer, theinternal surfaces of the reactor are dosed with a vapour or gascomprising ammonia, hydrazine or volatile organic amines. This dosingoperation is performed at low pressure conditions (0.05 to 100 mbar),between 100 and 350° C., typically at around 200° C. and during lessthan 10 minutes, typically around to 2 minutes. The flow of gas has tobe sufficient. For example, 90 mbar.sec is a good value for ammonia.After the dosing operation, a short pumping period of less than 5minutes is also respected before the deposition of the i-layer.

[0051] A nitrogen peak can be observed with a SIMS analysis at thetreated n-i interface, meaning that such a treatment has been used.Typically, the amount of nitrogen is higher than 10¹⁹ atoms.cm⁻³.

[0052] For both embodiments of the invention, it may be useful to deposeon the p-layer, after the above described treatments, a hydrogen-dilutedbuffer layer. This layer is obtained by PECVD of a mix of 10% silane and90% hydrogen. The plasma excitation frequency used is 40.68 MHz, thetemperature is 200° C., while the pressure is kept at 0.5 mbar, and thepower RF is applied at a level of 60 W. Such a layer alone has usuallyalready a beneficial effect on the boron cross contamination in thei-layer.

[0053] The method of the invention, according to both describedembodiments, offers the advantage to eliminate the boron contaminationwhile working with a single reactor. There is neither wasted pumpingtime nor loss of time due to transfer of the substrate out of thereactor for a cleaning step nor loss of time for reheating of thesubstrate which cooled down during the transfer. Moreover, apart fromsimpler and faster processes the single chamber approach bears thepotential of considerably simplified deposition systems as compared tomulti-chamber systems. It has to be noted that such methods allow toproduce a complete solar cell in only 30 minutes.

[0054] A person skilled in the art can easily adapt the above describedtreatments to a n-i-p solar cell in order to avoid phosphorcross-contamination after the deposition of n-doped layer.

[0055] Needless to say that the invention can also be applied to a anyjunction based on a p-doped or n-doped layer. The dosing can also beperformed by injecting the dosing compound directly in the gas feedingline.

1. A method for producing a semi-conducting device comprising at least alayer doped with a doping agent and a layer of another type deposited onsaid doped layer in a single reaction chamber, wherein the depositionsteps of said layers are separated by an operation for avoiding thecontamination by the doping agent of said another layer.
 2. The methodof claim 1, wherein said operation comprises a dosing of the reactionchamber with a compound able to react with the doping agent.
 3. Themethod of any of claims 1 and 2, wherein said operation comprises adosing of the reaction chamber with a vapour or gas comprising water,methanol, isopropanol or another alcohol.
 4. The method of any of claims1 and 2, wherein said operation comprises a dosing of the reactionchamber with a vapour or gas comprising ammonia, hydrazine or volatileorganic amines.
 5. The method of any of claims 3 and 4, wherein saiddosing is performed at around 0.05 to 100 mbar and between 100 and 350°C. for less than 10 minutes.
 6. The method of claims 1 to 6, wherein thedoped layer is a p-doped layer.
 7. The method of claims 1 to 6, whereinthe doped layer is a n-doped layer.
 8. The method of claim 6, whereinsaid operation is followed by the deposition of a buffer layer on thep-layer.
 9. The method of any of claims 2 to 8, wherein said dosing isfollowed by a pumping at high vacuum and between 100 and 350° C. forless than 5 minutes.
 10. A semi-conducting device comprising at least alayer doped with a doping agent and a layer of another type deposited onsaid doped layer, wherein the interface between said layers containstraces of oxygen as a result of a treatment for avoiding thecontamination of said another layer by the doping agent.
 11. Thesemi-conducting device of claim 10, wherein the content of oxygen ishigher than 10¹⁹ atoms.cm⁻³.
 12. A semi-conducting device comprising atleast a layer doped with a doping agent and a layer of another typedeposited on said doped layer, wherein the interface between said layerscontains traces of nitrogen as a result of a treatment for avoiding thecontamination of said another layer by the doping agent.
 13. Thesemi-conducting device of claim 12, wherein the content of nitrogen ishigher than 10¹⁹ atoms.cm⁻³.